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The FBI has obtained a warrant to search the emails found on a computer used by former Congressman Anthony Weiner that may contain evidence relevant to the investigation into Hillary Clinton's private email server, according to law enforcement officials.

One official said the total number of emails recovered in the Weiner investigation is close to 650,000 - though that reflects many emails that are not in any way relevant to the Clinton investigation.

Officials familiar with the case said, though, the messages include a significant amount of correspondence associated with Clinton and her top aide Huma Abedin, Weiner's estranged wife.

The agents investigating Clinton's use of a private email server knew early this month that messages recovered in a separate probe might be germane to their case, but they waited weeks before briefing the FBI Director, according to people familiar with the case.

Director James Comey has written that he was informed of the development last week, and he sent a letter to legislators the next day letting them know that he thought the team should take "appropriate investigative steps designed to allow investigators to review these emails".

That missive ignited a political firestorm less than two weeks before the election. Almost instantly, Comey came under intense criticism for his timing and for bucking the Justice Department's guidance not to tell Congress about the development.

And his announcement means that Clinton could have to contend with the news that the FBI has resumed its investigation of her use of a private email server - without any real clarity on if its investigators will actually find anything significant - up to and beyond election day.

People familiar with the case said they had known about the messages since soon after New York FBI agents seized a computer related to their investigation into Weiner, who is alleged to have exchanged explicit messages with a 15-year-old girl.

Abedin has told people that she is unsure how her emails could have ended up on a device she viewed as belonging to her husband, according to a person familiar with the investigation and civil litigation over the matter.

An announcement from the FBI in early October, when the emails were discovered, might have been less politically damaging for Clinton than one coming less than two weeks before the November 8 election.

It is also unclear what agents have been doing in the intervening time - for instance, whether they were trying to learn more about the emails before notifying Comey. An FBI spokesman declined to provide a statement.

Comey wrote in his letter to Congress, "We don't know the significance of this newly discovered collection of emails," and federal law enforcement officials have said that investigators on the Clinton email team still had yet to thoroughly review them. They needed a warrant to do so.

Comey in July announced that he was recommending the investigation into Clinton's use of a private email server while she was Secretary of State be closed without charges.

Investigators will be looking at whether the newly uncovered emails contain classified information or other evidence that could help advance the Clinton email probe. It is possible, though, that the messages could be duplicative of others already recovered elsewhere or that they could be a collection of benign, personal notes.

Legislators on both sides of the political aisle are likely to raise questions about why the team investigating Clinton's private email took so long to brief Comey. Clinton and her backers have pushed aggressively for the bureau to release more information about its findings and criticised the agency for making its work public without knowing more.

A Washington Post-ABC News tracking poll found that more than 6 in 10 likely voters said the FBI's announcement would make no difference in their vote. A little more than 3 in 10 said the news made them less likely to support Clinton, though about two-thirds of those were Republicans or Republican-leaning independents.